1. Field of the Invention
The present invention relates to an error signal detecting device, in irradiating an object with a light beam concentrated as a spot, for detecting a positional error of said spot with respect to said object along the direction of the optical axis (so-called focusing error), or a positional error in a direction perpendicular to the optical axis (so-called tracking error), and in particular such error signal detecting device adapted for use in an optical information record/regenerating apparatus for optically recording or regenerating information.
2. Related Background Art
In an optical information record/regenerating apparatus, a light beam irradiates an optical recording medium such as an optical disk or a magnetooptical disk, and a reflected or transmitted light is utilized for focus control and tracking control by driving a servo system. In the following there will be explained a focus control process in a conventional optical information record/regenerating apparatus.
FIG. 1 is a schematic view of an example of conventional optical information record/regenerating apparatus, wherein a light beam emitted by a light source 37 such as a semiconductor laser is converted into a parallel beam by a collimating lens 38, then guided through a beam splitter 39 and concentrated as a spot on a recording medium 1 such as an optical disk by means of an objective lens 2. The light reflected by said recording medium 1 is guided again through the objective lens 2, then reflected by the beam splitter 39 and focused onto a photosensor 4 by a condenser lens 3. By modulating said light beam with an information signal, said information is recorded on said recording medium 1 in the form of pit patterns which are optically detectable for example by changes in the reflectance. The information thus recorded can be regenerated by irradiating said recording medium 1 with an unmodulated light beam and photoelectrically converting the reflected light, modulated by said pit patterns, by means of said photosensor 4. Also from the output of said photosensor 4 there can be detected a focusing error signal and a tracking error signal.
FIG. 2A is a schematic view of a light-receiving optical system, for showing the principle of focus error detection in the above-explained optical information record/regenerating apparatus, and FIG. 2B is a schematic plan view of a two-division photosensor employed in said optical system. FIG. 3A is a chart showing the relationship between the distance of the optical system and recording medium and the outputs a.sub.0, b.sub.0 of said two-division photosensor, and FIG. 3B is a chart showing the relationship between said distance of deviation and (a.sub.0 - b.sub.0).
In FIG. 2A, the light reflected or transmitted by the recording medium 1 is transmitted by the objective lens 2 and is concentrated by the condenser lens 3. The converging point is at f if the objective lens 2 is spaced by a predetermined distance from the recording medium 1, but moves toward f' or f" respectively if the objective lens 2 is closer to or farther from the recording medium 1.
The photosensor 4 is positioned slightly closer to the condenser lens 3 than the moving range of said converging point. As shown in FIG. 2A, the photosensor 4 is divided into a circular central portion 5a of a predetermined area and a peripheral portion 5b, respectively releasing electric signals a.sub.0, b.sub.0 corresponding to the amounts of incident light. Consequently a change in the distance between the objective lens 2 and the recording medium 1 causes a change in the amounts of light entering the central portion 5a and the peripheral portion 5b, and the focus can be controlled by said change.
The photosensor 4 is so regulated that the central portion 5a and the peripheral portion 5b provide same outputs a.sub.0, b.sub.0 as shown in FIG. 3A when the objective lens 2 is at the predetermined distance, i.e. zero deviation, from the recording medium 1. In such a state, if the objective lens 2 moves closer than said predetermined distance (negative deviation), the converging point moves toward f' to increase the irradiated area on the photosensor 4, whereby the output a.sub.0 of the central portion 5a decreases while the output b.sub.0 of the peripheral portion 5b increases. 0n the other hand, if it moves farther than said predetermined distance (positive deviation), the converging point moves toward f" to decrease the irradiated area on the photosensor 4, whereby the output a.sub.0 of the central portion 5a increases while that b.sub.0 of the peripheral portion 5b decreases. Consequently an error signal (a.sub.0 - b.sub.0) indicating the difference of two outputs a.sub.0, b.sub.0 assumes a form shown in FIG. 3B, and can be used for focus control in a range where the deviation is small.
However, as will be understood from FIG. 3B, said error signal (a.sub.0 - b.sub.0) does not vary linearly even when the deviation is small, and is smaller when the objective lens 2 is close to the recording medium 1 than when the objective lens 2 is far from the recording medium 1. For this reason stable focus control cannot be achieved in the conventional optical information record/regenerating apparatus.
The unstability in control resulting from non-linearity of the error signal exists also in the well known focus error detection utilizing a knife edge, or in the tracking control for causing the light beam to following a recording track on the recording medium.
In addition to the foregoing drawback, the conventional technology has been associated with another drawback explained in the following.
In general the power of the light beam is stronger at recording than at regenerating, and, in case of recording on a disk-shaped recording medium, the power has to be made stronger in the external portion than in the internal portion, in consideration of a difference in the peripheral speed. In this manner the power of the light beam is not constant but is varied by various conditions.
Consequently the intensity of the light reflected or transmitted by the recording medium 1 varies likewise, and the outputs a.sub.0, b.sub.0 of the photosensor 4 varies correspondingly. In order to achieve stable focus control and tracking control under such condition, the gain of the servo mechanism has to be automatically regulated so as to obtain an averaged servo signal regardless of the change in the power of light beam.
For this reason, as proposed in the Japanese Patent Publication No. 56138/1982, a voltage-controlling amplifier or a dividing circuit is provided in the loop of the automatic focus control system, and the focus control is conducted by driving the objective lens with a servo circuit of a gain inversely proportional to the amount of incident light. In such an operation the voltage-controlling amplifier or the dividing circuit is usually operated in a linear characteristic portion of transistors.
However, if the difference in the amount of light is large between the recording and regenerating operations, or if the reflectance of the recording medium fluctuates considerably, the transistors have to be operated in non-linear range, so that it has been difficult to achieve acceptably stable control corresponding to the change in the amount of light.
As explained in the foregoing, the conventional optical information record/regenerating apparatus has been unable to achieve stable focus and tracking controls because of the facts that the error signal does not vary linearly and that the linearity of characteristics of the signal processing circuit is easily affected when the amount of incident light varies significantly.